The present invention relates to an electromagnetic flowmeter and, more particularly, to a compact electromagnetic flowmeter which has a small distance between both end faces thereof and which is easily connected to pipes to measure with high precision a flow speed of a fluid in the pipes.
Generally, electromagnetic flowmeters measure a flow rate of any electrically conductive fluid with high precision. The measuring tubes of electromagnetic flowmeters, through which the liquid flows, have no internal projections, so that pressure loss does not occur, resulting in convenience. However, flowmeters of this type are heavy and cannot be easily connected to pipes.
In order to eliminate the above drawbacks, a flangeless electromagnetic flowmeter is proposed wherein a pair of excitation coils are radially opposed on an axis perpendicular to a longitudinal axis of a cylindrical ferromagnetic ring; a plastic cylinder is concentrically disposed in the ring to form a measuring tube, one end of which has a smalldiameter flange and the other end of which has a flange fitted into the ring; a pair of electrodes are disposed normally to a line which is perpendicular to both the axis of the pair of excitation coils and the axis of the ring; and a filler such as an epoxy resin is filled in a space between the ring and the measuring tube to form an annular pressure chamber.
However, since the two ends of the electromagnetic flowmeter of this type are respectively defined by an end face of the annular pressure chamber and the flange of the plastic measuring tube, both ends of the flowmeter cannot be magnetically sealed. The following drawbacks are thus presented:
(1) The material of the flange at the end of the pipe to which the electromagnetic flowmeter is connected cannot be freely selected. A nonmagnetic insulator (nonmagnetic material, such as a resin and austenitic stainless steel, having a relatively high electrical resistance) can be used. However, a magnetic flange cannot be used. PA0 (2) Since the magnetic poles are fairly separated from the magnetic strip, the magnetic fluxes are not extended. Thus, large excitation coils are required to make the flux distribution ideal. PA0 (3) A distance between the two end faces of the electromagnetic flowmeter must be sufficiently greater than the outer diameter of an iron core of the excitation coil. This is because the end faces of the flowmeter must not be influenced by the magnetic flux. PA0 (4) The material of the bolts for fastening the electromagnetic flowmeter at the flanges of the pipes cannot be freely selected. Although austenitic stainless steel and plastic bolts can be used, magnetic bolts cannot be used. PA0 (5) In a structure which has a ferromagnetic strip as a magnetic path in place of the ferromagnetic ring to interconnect the pair of excitation coils, the magnetic flux leaks around the strip and an error occurs in a flow rate measuring value when the flowmeter is installed near a ferromagnetic structure such as a steel tank or steel piping. Further, the flowmeter tends to be influenced by any external magnetic field. The flowmeter cannot be installed near a power generator or a motor. Thus, the installation location of the flowmeter is limited. PA0 (6) Since the pair of excitation coils are disposed between the bores for fastening bolts, the diameter of the electromagnetic flowmeter is of the order of 100 mm. If the diameter is increased to more than 100 mm, the number of bores must be increased to comply with the standards. As a result, the distance between the bores is small and the excitation coils cannot be disposed in the narrow space.